1. Field of the Invention
This invention relates to target tracking radar, particularly to high PRF pulse doppler tracking radar.
2. Description of the Prior Art
In the prior art, a high PRF pulse doppler radar is used in the search mode to detect a target. The target velocity is determined by the location of the target reflected signal in the doppler filter bank in the search receiver of the radar. The coarse range of the target is determined in the search receiver by utilizing the radar in an FM ranging mode. The high PRF pulse doppler radar is then operated in the track mode whereby the track channel, using a narrow band pass filter, is tuned to the target's doppler frequency. If the target reflected signals are not centered exactly within the receive gate of the radar receiver, there will be loss in target signal power which will reduce the signal to noise ratio. The non-alignment of target reflected signals with respect to the receive gate of the radar receiver is referred to as eclipsing loss. If target reflected signals are received outside the receive gate, the eclipsing loss is total, which is also known as a blind range since the arrival of target reflected signals with respect to the receive gate are dependent upon the range of the target and the PRF of the radar. As a consequence, multiple PRF's are used to reduce eclipsing losses in the track mode.
A typical high PRF waveform would have a 45% transmit time, followed by a 10% receiver recovery time, and a 45% receive time for one interpulse period. A typical PRF would be 250 KHz.
During track, a single PRF is used until the target reflected signal is lost or no longer in the receive gate. The loss of signal may be due to eclipsing loss or due to target scintillation where the aspect of the target results in a weaker target reflected signal or signal fade-out. The loss in target reflected signals in the track mode is assumed due to eclipsing loss or a blind range. The unambiguous range for a typical PRF of 250 KHz is about 0.32 nautical miles resulting in blind ranges at multiples of 0.32 nautical miles. Upon loss of target reflected signals a second PRF is used and if the target reflected signals are not reacquired in the receive gate after some period of time, another PRF is used. A typical number of PRF's is 3 to 5 for a radar in the track mode. After sequentially trying these PRF's several times without reacquiring the target, the track receiver is tuned to see if the target has changed doppler frequency due to a change in radial velocity with respect to the radar antenna. If the target is still not reacquired, the radar is returned to the search mode.
Several methods to reduce the effects of blind ranges during tracking have been used such as selecting subsets of PRF's as a function of coarse target range or selecting PRF switching sequences for nominal target velocities. Certain PRF values, however, result in spurious signals due to modulation of the main beam clutter by the receive gate which signals can fall within the tracking pass band and interfere with the tracking performance of the radar. The PRF values which can be used for tracking must exclude these PRF values. As a result, use of multiple PRF's to reduce blind ranges is far from perfect. For example, a typical tracking radar using 5 PRF's requires 5 to 10 dB of additional signal to noise, depending upon the target range, to overcome eclipsing loss.
In view of the prior art, it is therefore desirable to eliminate eclipsing loss or loss of the target reflected signal in the receive gate by continuously varying the PRF of the transmitted high PRF signal to keep the target reflected signal within the receive gate of the radar receiver. The radar receiver therefore receives most or all of the target reflected signal on a continuous basis during track. The effective signal to noise ratio of the target reflected signal is increased which is important for tracking long range targets. In addition, the target reflected signal is not lost periodically in the receive gate of the radar receiver which would require reacquisition of the signal which may lead to periods of signal loss and to possible track loss.
It is desirable when the target reflected signal is weak or lost due to target scintillation to continue to vary the PRF of the transmitted high PRF signal to keep subsequent target reflected signals within the receive gate of the radar receiver to avoid signal loss in the receive gate which would require reacquisition of the target reflected signal which may lead to unnecessary periods of signal loss and to possible track loss.
It is desirable to reduce the level of spurious signals caused by intermittent receiver operation below the receiver noise level of the radar receiver to permit utilization of all PRF values within a predetermined upper and lower PRF limit for tracking a target.